The present application relates to propellant compositions e.g. rocket propellant or gas generator compositions, that have a binder formed from compositions of thermoplastic ethylene co-polymers, especially ethylene/vinyl acetate polymers (EVA) and other related polyolefins. In embodiments, the binder is a composition of cross-linkable thermoplastic ethylene copolymer, especial cross-linkable ethylene/vinyl acetate copolymer or related cross-linkable polymer. An example of a cross-linkable polymer is silane-grafted EVA.
The original black powder rocket propellants were replaced In the early 1900""s with propellants based on nitrocellulose and nitroglycerin. Subsequently propellants were developed that were based on a fuel oil, a binder e.g. asphalt, and an oxidizer e.g. potassium perchlorate. Polysulphide fuel binders that could be cast and cured at cool temperatures, mixtures of ammonium perchlorate, polyester and styrene cured by cumene hydroperoxide and compositions based on polyvinyl chloride plastisols were also developed.
A number of polybutadiene materials, including in particular polybutadiene acrylonitrile, carboxy terminated polybutadiene and hydroxy-terminated polybutadiene have also been developed and undergone commercial use. In particular, repellent compositions using polybutadiene acrylonitrile (PBAN) as binder hive been developed and are used in a number rocket systems, including the solid rocket boosters for the Space Shuttle. Propellant compositions using hydroxy-terminated polybutadiene (HTPB) are also known and in use. It is understood that systems utilizing thermoset polymers such as PBAN and HTPB exhibit relatively long curing times (several days) unless promoted through heat and/or catalysis, and that pot-life suffers accordingly, and may be as short as about 20 minutes. In most cases, propellant compositions using the above binder systems and related systems require the use of toxic chemicals such as epoxides, dilsocyanates or aziridines as curing agents. In addition, plasticizers e.g. ethyl hexyl acrylate or di-octyl adipate may be used, which are also known to exhibit toxicological properties. In addition to safety considerations during manufacture of the propellants, the cost of many of these constituents is relatively high.
The shelf life of some of these constituents, such as epoxides and dilsocyanates, tends to be short. Special handling e.g. freezing or refrigeration and/or inert gas blanketing, is required to extend their useful life, which further increases the overall cost of the propellants. Of greater concern is the potential for allergic reactions and the consequent need for special handling in order to protect persons handling the compositions.
Propellant compositions offering greater flexibility, less stringent handling requirements and less lead time in fabrication would be useful.
Propellants formed from compositions of thermoplastic polymers, and methods for the manufacture thereof, have now been found.
Accordingly, an aspect of the present invention provides a solid propellant composition comprising a binder and at least 65% by weight of a material selected from the group consisting of oxidizer and crystalline high explosive, said binder being selected from the group consisting of a thermoplastic ethylene copolymer and a cross-linkable thermoplastic ethylene copolymer.
In preferred embodiments of the invention, the ethylene copolymer is ethylene/vinyl acetate copolymer or other ethylene/vinyl alkanoate copolymer or the copolymer is selected from an ethylene/ethyl acrylate copolymer, ethylene/methyl acrylate copolymer or ethylene/butyl acrylate copolymer, a copolymer of ethylene with acrylic acid or methacrylic acid, an ionomer thereof and a copolymer of ethylene with an acrylic or methacrylic acid ester.
In other embodiments, the crystalline high explosive is selected from the group consisting of cyclotetramethylenetetranitramine, cyclotrimethylenetrinitramine and hexanitrohexaazaisowurtzitane and the oxidizer is selected from ammonium perchlorate, ammonium nitrate and potassium perchlorate, especially ammonium perchlorate.
In further embodiments, the binder is cross-linkable ethylene/vinyl acetate copolymer, said copolymer having a moisture crosslinkable monomer, especially in which the moisture crosslinkable monomer is selected from vinyl trimethoxysilane and vinyltriethoxysilane. Alternatively, the binder is a silane-grafted ethylene/vinyl acetate copolymer.
In additional embodiments, the material is oxidizer.
In preferred embodiments, the composition contains at least 70% by weight of oxidizer, especially 75-90% by weight of oxidizer.
In other embodiments, there is at least one of an energetic, a ballistic modifier and a modifier, said energetic being selected from the group consisting of aluminum, magnesium and aluminum/magnesium alloys, said ballistic modifier being selected from the group consisting of oxides of iron, copper, chromium and magnesium and calcium carbonate and said modifier being selected from the group consisting of a titanate, a zirconate and an aluminate.
In further embodiments, there is at least one of an additive selected from opacifiers; stabilizers; metal de-activators; anti-oxidants; flame colorants; or an agent that modifies the processing, performance, mechanical properties, storage stability or shelf life of solid propellant systems. Preferably, the stabilizer is selected from zinc oxide, nickel oxide and triacetin, and the flame colorant is selected from salts of strontium, barium, sodium and lithium. In addition, the oxidizer is preferably a mixture of particle sizes selected from coarse, medium, fine and ultra fine particles, said coarse particles being 400-600 micron, said medium particles being 50-200 micron, said fine particles being 5-15 micron and said ultrafine particles being submicron to 5 micron.
In embodiments, the composition is a gas generator propellant or a rocket propellant.
In preferred embodiments, at least 75% by weight of the material is oxidizer.
In further embodiments the propellant composition additionally comprising a plasticizer that is solid or semi-solid at 20xc2x0 C., and an additive to increase one or more of elongation, adhesion and tack. The solid or semi-solid plasticizer is preferably selected from microcrystalline wax, macrocrystalline wax, an oxidized hydrocarbon polyolefin and a polyketone wax and the additive selected from a hydrogenated hydrocarbon resin and a derivative of a rosin.
In embodiments, the binder composition contains 35-65% by weight of copolymer, 10-30% by weight of solid or semi-solid plasticizer and 20-40% by weight of said additive. The propellant composition may contain 10-20% of binder.
In other embodiments, the propellant composition comprises 50-90% by weight of oxidizer and 5-20% by weight of ethylene/vinyl acetate copolymer, the remainder of such composition comprising at least one of said crystalline high explosive, plasticizer, energetic, ballistic modifier and other propellant components.
Another aspect of the invention provides a method of manufacture of a propellant composition comprising:
(a) preparing a pre-propellant composition of ethylene copolymer; and
(b) admixing the pre-propellant composition with a material selected from oxidizer and crystalline high explosive such that the resulting composition has at least 65% by weight of said material.
In preferred embodiments of the method, the propellant composition obtained in (b) it formed into propellant grains by an extrusion process and/or the grain is consolidated into final form under mechanical, pneumatic or hydraulic pressure or centrifugal force.
In embodiments, the extrusion process utilizes a cooling cycle to cool the propellant grain.
In other embodiments, the pre-propellant composition is prepared by melt blending.
In embodiments, the propellant composition obtained in (b) is formed into fuel grains in a ram extruder.
In further embodiments, there is a cold cycle to cool the propellant grain so obtained.
In other embodiments, the pre-propellant contains at least one of an energetic and ballistic modifier said energetic being selected from the group consisting of aluminum, magnesium and aluminum/magnesium alloys, and said ballistic modifier is selected from the group consisting of oxides of iron, copper, chromium and magnesium, and calcium carbonate. The pre-propellant may contain at least one additive selected from opacifiers; stabilizers; metal de-activators; anti-oxidants; flame colorants; or an agent that modifies the processing, performance, mechanical properties, storage stability or shelf life of solid propellant systems.
In preferred embodiments, the oxidizer is a mixture of particle sizes selected from coarse, medium, fine and ultra fine particles, said coarse particles being 400-600 micron, said medium particles being 50-200 micron, said fine particles being 5-15 micron and said ultra fine particles being submicron to 5 micron.
A further aspect of the intention provides a solid propellant composition comprising a binder and at least 50% by weight of an oxidizer, said binder being an ethylene-vinyl alkanoate copolymer, especially ethylene/vinyl acetate copolymer. The copolymer of the binder may have a moisture crosslinkable monomer, especially vinyl trimethoxysilane or vinyltriethoxysilane. The binder may be a silane-grafted ethylene/vinyl acetate copolymer.